The Miocene saw a change in global circulation patterns due to slight
position changes of the continents and globally warmer climates.
Conditions on each continent changed somewhat because of these positional
changes, however it was an overall increase in aridity through mountain
building that favored the expansion of grasslands. Because the positions of
continents in the Miocene world were similar to where they lie today,
it is easiest to describe the plate movements and resulting changes in
the paleoclimate by discussing individual continents.

In North America, the Sierra Nevada and Cascade Mountain ranges formed, causing
a non-seasonal and drier mid-continent climate. The increasing occurrences of
drought and an overall decrease in absolute rainfall promoted drier climates.
Additionally, grasslands began to spread, and this led to an evolutionary
radiation of open habitat herbivores and carnivores. The first of the major
periods of immigration via the Bering land connection between Siberia
and Alaska occurred in the middle of the Miocene, and by the end of the Miocene
the Panama isthmus had begun to form between Central and South America.

Plate tectonics also contributed to the rise of the Andes Mountains in
South America, which led to the formation of a rain shadow effect in the
southeastern part of the continent. The movement of the plates also
facilitated trends favoring non-desert and highland environments.

In Australia, the climate saw an overall increase aridity as it continued to
drift northwards, though the continent went through many wet and dry periods.
The number of rainforests began to decrease, to be replace by dry forests and
woodlands. The vegetation began to shift from closed broad-leaved forests to
more open, drier forests as well as grasslands and deserts.

Eurasia also experienced increasing aridification during the Miocene. Extensive
steppe vegetation began to appear, and the grasses became abundant. In southern
Asia, grasslands expanded, generating a greater diversity of habitats.
However, southern Asia was not the only area to experience an increase in
habitat variability. Southern Europe also saw an increase in grasslands, but
maintained its moist forests. Although most of Eurasia experienced increasing
aridity, some places did not. The climate in some Eurasian regions, such as
Syria and Iran, remained wet and cool.

During the Miocene, Eurasia underwent some significant tectonic rearrangements.
The Tethys Sea connection between the Mediterranean and Indian Ocean was
severed in the Mid-Miocene causing the increase in aridity in Southern Europe.
The Paratethys barrier, which isolated Western Europe from the exchange of
flora and fauna, was periodically disrupted, allowing for the migration of
animals. Additionally, faunal routes with Africa were well established and
occasional land bridges were also created.

Africa also encountered some tectonic movement, including rifting in East
Africa and the union of the African-Arabian plate with Eurasia. Associated
with this rifting, a major uplift in East Africa created a rain shadow effect
between the wet Central-West Africa and dry East Africa. The union of
the continents of Africa and Eurasia caused interruption and contraction of
the Tethys Sea, thereby depleting the primary source of atmospheric moisture
in that area. Thus rainfall was significantly reduced, as were the moderating
effects of sea temperature on the neighboring land climates. However, this
union enabled more vigorous exchanges of flora and fauna between Africa and
Eurasia.

Antarctica became isolated from the other continents in the Miocene, leading
to the formation of a circumpolar ocean circulation. Global ocean and
atmospheric circulation were also affected by the formation of this circumpolar
circulation pattern, as it restricted north-south circulation flows.
This reduced the mixing of warm, tropical ocean water and cold, polar water
causing the buildup of the Antarctic polar ice cap. This accelerated the
development of global seasonality, aridity, and enhanced global cooling.

Find out more about the Tertiary paleontology and geology of North America at the Paleontology Portal.